Interferometer type receivers are widely used for locating, for example, hostile radar emitters from, for example, a moving platform. Typical RF interferometer systems include a plurality of antennas with which an emitter is located by utilizing the phase differences between the signals arriving at the different antennas. In the design and development of such systems the question is raised, of course, as to how they should be tested. Previously, testing such systems in a laboratory has been, at best, awkward. It has been necessary to simulate the different phase shifts to be seen by each antenna using differing lengths of transmission line. Usually, only one set of lines was used simulating only one emitter at a predetermined azimuth.
Developments in the digital generation of signals, particularly in the 0 to 300 MHz range have created important new possibilities for laboratory testing of interferometer type receiver systems. It is now possible using digital techniques to generate signals wherein a given phase of an output signal may be started at any arbitrary point and changed virtually instantaneously. More than one such device can be driven from a common reference frequency, and the phase of each output can be set independently while being locked to the phase of the common reference source. These devices are known as direct digital frequency synthesizers ("DDFS") and can be incorporated in microwave synthesizers allowing virtually any frequency to be generated having the phase locking characteristics described.